In thermal crackers for producing ethylene, a hydrocarbon material such as naphtha, natural gas or ethane is fed to a reactor tube as a fluid mixture (about 700.degree. K in temperature) with steam and heated to a specified temperature (about 1090.degree. K) with heat supplied from outside while being passed through the tube at a high velocity (e.g., about 0.1 to 0.8 second to flow through the reactor tube when the tube is about 10 to about 13 m in length), whereby olefins such as ethylene, propylene and the like are produced by thermal cracking.
It is required for the thermal cracking operation to effect efficient heat transfer to the fluid passing through the tube at a high velocity and to rapidly heat the fluid to the reaction temperature range up to the central portion thereof. At the same time, unnecessary heating to a high temperature beyond the cracking temperature range must be avoided to the greatest possible extent. The reason is that if the hydrocarbon material is retained in a high temperature range beyond the cracking temperature range for a long period of time, the material becomes converted into lighter fractions (for example, methane and free carbon) to excess or the cracked product undergoes polycondensation or like reaction, consequently reducing the yield of the desired product greatly while permitting the deposition of free carbon on the tube inner surface to result in the disadvantage of necessitating more frequent decoking.
To assure the reactor tube of an improved heat transfer efficiency and to heat up the fluid inside the tube rapidly, it is conventional practice to decrease the bore diameter of the tube (for example, to not larger than 40 mm) and increase the heat transfer surface area relative to the content volume. Further heat exchange tubes have been proposed which, as shown in FIG. 12, are formed with a corrugated inner wall surface, with ridges and furrows extending helically or in parallel to the tube axis, thus shaped ingeniously in cross section and thereby given an increased heat transfer area (Unexamined Japanese Patent Publications No. 173022/1983 and No. 127896/1989).
Although a decrease in the diameter of the reactor tube is effective for transferring heat to the fluid inside the tube more efficiently, it is then required that an increased number of reactor tubes be installed in the cracker to compensate for the decrease in the tube diameter if the apparatus is to remain unchanged in ethylene production capacity. This renders the apparatus more cumbersome to operate and maintain while the decrease in the tube diameter entails the drawback that free carbon is liable to become deposited on the tube wall in a shortened period of time.
The reactor tube of FIG. 12 wherein the inner wall surface is corrugated merely achieves an improvement in heat transfer efficiency generally corresponding to the increase in the tube wall inner surface area and can not be expected to attain a remarkably improvement in any other function over usual tubes (having a smooth inner wall surface with no corrugation).
In view of the foregoing problems, the present invention provides a novel heat exchange tube which achieves a remarkably improved efficiency in transferring heat to the fluid flowing therethrough without necessitating a reduction in its inside diameter and which is rapidly heatable for the fluid to a required temperature up to its central portion, the tube being adapted for use in thermal crackers for producing ethylene to make it possible to greatly increase the production capacity thereof and render the apparatus compact in design and easy to operate and maintain.